I thougt "If I do not " and "if I don't" is exactly the same thing.
-faulu mtihani means "pass". -faulu by itself means "to succeed". Kamusi ya Kiswahili Sanifu: faulu - pata yaliyokusudiwa; shinda katika jambo; pata muradi; fanikiwa; fuzu.
"Sitafaulu" translates to not succeeding, "I will not pass" is "sitapita"!
Well I guess I can partly agree with you, since the word "mtihani" does not appear in the given sentence. On the other hand, I think that -"faulu mtihani" and also "-fuzu mtihani" both mean "to pass an exam."